In the manufacture of printed circuit boards and cards, a dielectric sheet material is employed as the substrate. A conductive circuit pattern is provided on one or both of the major surfaces of the substrate.
As described in, for instance, U.S. Pat. No. 3,689,991 and Tummala et al, "Micro Electronics Packaging Handbook", pages 409-435, Van Nostrand Reinhold, flexible polymeric films can be used as carriers in the packaging of semiconductor chips such as in the so-called TAB (Tape Automated Bonding) procedure. To date, the primary polymeric material employed for such has been polyimides. One procedure used for employing polyimide as the dielectric and/or circuit carrier for flexible circuits involves spray coating or roller coating polyamic acid onto a sheet of metal (such as stainless steel or aluminum). The film is then cured or imidized, resulting in a film which is fully or substantially fully cured. The metal which the polyimide is on can be imaged, removed or maintained. On top of the polyimide, three layers of metal are generally deposited such as by either evaporation or sputtering. The metal conductors are chromium or nickel, followed by a layer of copper, followed by a layer of chromium or nickel. By means of photolithographic operations, this metal is imaged into circuits. Depending on the use of the circuit, the cured polyimide may or may not be imaged either before or after the formation of the circuit.
In addition, flexible circuits have also been fabricated using free-standing polymeric films such as polyimides onto which metal layers are vacuum-deposited, laminated or glued. The metal circuit pattern is defined by using a photoresist pattern to either act as a plating mask or act as a mask for subtractive etching of the metal layer. Via holes of the polymer film can be made by drilling, punching or etching.
In addition, the selective formation of holes in insulating polymeric films to provide openings or vias therein is important for various electronic uses of dielectric polymeric films. For instance, in the packaging of semiconductor chips, polymers such as polyamic acid films have often be coated onto substrates and then cured either chemically or thermally.
In a number of these situations it is necessary to form vias into the polymeric layer to allow for electrical connections to be made between the different layers of metallurgy. In order that the interconnection be as accurate as possible, it is necessary that the polymeric films resist distortion of the desired pattern and withstand attack from other wet processing chemicals.
For instance, in the formation of multi-layer substrates for mounting chips, it is necessary to electrically connect some of the conductors in the upper or second layer of metalization to some of the conductors on the lower or first layer of metalization. In order to do so, the polymeric material must have vias formed therein to allow for metal connection between the upper and lower levels of metallization in connection to a chip and/or board.
Fluorinated polymeric materials such as polytetrafluoroethylene and polytrifluoromonochloroethylene are attractive candidates for advanced electronic packaging applications because of their very low dielectric constants, excellent stability, low solvent/moisture absorption and excellent thermal stability. However, the fluorinated polymeric materials have very poor coefficient of thermal expansion and therefore must include modifiers therein to achieve a coefficient of thermal expansion suitable for use as a substrate for electronic devices.
It has been suggested to provide compositions of polytetrafluoroethylene materials with certain fillers such as glass or ceramic microparticles to achieve improved dimensional stability and low thermal expansion coefficient (CTE) without a concomitant loss in the low dielectric properties in the polymeric material. In addition, for the most part these composites also include glass fibers therein. Along these lines, see U.S. Pat. Nos. 4,335,180 and 4,849,284.
However, the compositions suggested in the prior art require an amount of filler in excess of the polymeric material and/or the presence of fibers in addition to the relatively small particle size fillers. The fillers employed have mean particle diameters of 10-15 microns and films formed therefrom are a minimum of 1.5 mils thick.
The composites suggested in the prior art are not entirely satisfactory since difficulties are encountered in fabricating vias therein and laser drilling cannot be employed readily because of the relatively large size of the particles. Moreover, when glass fibers are present such tend to remain in the through holes.
Furthermore, the prior art suggestions result in compositions prior to film formation that are relatively doughy in consistency and therefore quite difficult to work with.